The overall objective of this proposal is to further define the regulation of proteolysis in premature neonates. As the balance between proteolysis and protein synthesis ultimately determines whether protein accretion and growth will occur, identifying potential regulators of proteolysis may serve as a means to maximize protein accretion in this nutritionally vulnerable population. Exogenous insulin is currently being used in extremely premature neonates to promote growth, although the effect of insulin on protein metabolism is unknown. In addition, parenteral nutrition solutions are frequently used in sick and premature neonates who are unable to tolerate enteral feeding. The quantity and quality of amino acids provided to these neonates is likely to be a critical factor in maximizing protein accretion. The specific aims of this proposal are to: 1)determine how a continuous insulin infusion affects proteolysis in the extremely premature neonate 2)evaluate the extent to which graded amino acids suppress proteolysis in premature infants and 3)determine whether adding tyrosine and cysteine (thought to be essential amino acids in premature infants) to currently available amino acid solutions will further improve rates of protein accretion. Each of these specific aims will be pursued using the stable isotope tracers of the essential amino acids leucine and phenylalanine, whose endogenous rate of appearance is reflective of whole body proteolysis. Performing these studies will provide insight into the physiology of protein accretion, and thus the growth process. Ultimately, this information may serve as the basis for improved nutritional management of premature infants.